1. Field of the Invention
The present invention relates to a viewing optical system with an image-blur correcting optical system which can reduce the occurrence of image blur caused by hand shake.
2. Description of the Prior Art
Regarding a viewing optical system for a binocular and the like, many viewing optical systems in which an image-blur correcting optical system is moved in a direction perpendicular to the optical axis in accordance with an amount of hand shake have been proposed. For example, such viewing optical systems are disclosed in U.S. Pat. Nos. 6,226,123, 6,046,853, 6,072,630, 6,118,585, 6,198,575 and Japanese Unexamined Patent Publication No. Hei-11-264942.
The objective optical system of the above-mentioned viewing optical system with the image-blur correcting optical system is, in many cases, constituted by a two-lens-element arrangement, i.e., a positive lens element and a negative lens element. Furthermore, in the objective optical system constituted by three or more lens elements, it is often the case that an aspherical surface is utilized in the image-blur correcting optical system. The main purpose of using an aspherical surface in the image-blur correcting optical system is to reduce deterioration on aberrations caused by the driving of the image-blur correcting optical system in a direction perpendicular to the optical axis. However, if the aperture of the objective optical system is increased, the aperture of the image-blur correcting optical system also increases, so that the manufacture of a lens element with the aspherical surface becomes difficult and expensive.
The present invention provides a large-aperture viewing optical system with an image-blur correcting optical system, which achieves easy viewing therethrough, and adequately corrects aberrations without utilizing an aspherical surface.
As an aspect of the present invention, there is provided a viewing optical system including an objective optical system and an eyepiece optical system. The objective optical system includes a positive first meniscus lens element having the convex surface facing toward the object, a positive second meniscus lens element having the convex surface facing toward the object, a negative third lens unit, and a positive fourth lens element, in this order from the object. The fourth lens constitutes an image-blur correcting optical system which is movable in a direction perpendicular to the optical axis of the viewing optical system. The viewing optical system satisfies the following conditions:
0.5 less than |tan 1xc2x0/(xcfx861-4xe2x88x92xcfx861-3)| less than 5(mm)xe2x80x83xe2x80x83(1) 
0.5 less than SF4 less than 1.0xe2x80x83xe2x80x83(2) 
wherein
xcfx861-4 designates the combined power from the positive first meniscus lens element to the positive fourth lens element;
xcfx861-3 designates the combined power from the positive first meniscus lens element to the negative third lens unit; and
SF4 designates the shape factor of the positive fourth lens element defined as SF4=(r2+r1)/(r2xe2x88x92r1);
r1 designates the radius of curvature of the object-side surface of the positive fourth lens element; and
r2 designates the radius of curvature of the image-side surface of the positive fourth lens element.
The viewing optical system further satisfies the following conditions:
1.2 less than xcfx861-2/xcfx861-4 less than 2.0xe2x80x83xe2x80x83(3) 
xe2x80x83xe2x88x924.0 less than xcfx863/xcfx861-4 less than xe2x88x922.0xe2x80x83xe2x80x83(4)
0.10 less than d4xc3x97xcfx861-4 less than 0.17xe2x80x83xe2x80x83(5) 
wherein
xcfx861-2 designates the combined power of the positive first meniscus lens element and the positive second meniscus lens element;
xcfx863 designates the power of the negative third lens unit;
d4 designates the distance along the optical axis between the positive second meniscus lens element and the negative third lens unit; and
The negative third lens unit can be constituted by a single lens element or cemented lens elements.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-250386 (filed on Aug. 21, 2001) which is expressly incorporated herein in its entirety.